I went back and read your very first thread that you posted. Your h has a history of having affairs, both PA and EA, throughout his marriage. Leopards do not change their spots, as one poster posted.
I think your h never truly warmed up to the idea of remaining in the relationship from several years ago. I think he's been biding his time and now that there is someone else in the picture, he's going to use her as an "exist affair" to leave the marriage. I honestly do not think that this man was very sincere in trying to make it work. He may have said and done some of the "right" things that he thought you wanted to hear, but his heart hasn't been in trying to work on the issues that may have come up in the second marriage.
His issues run deep and unfortunately, he's a repeat offender when it comes to affairs. I know you love this man, but you've got to let him go and focus on you, your health and your child. If he straightens himself out and wants to reconcile, then he would need IC and prove himself to be honest, transparent in all things and earn your trust.
Let him go, let God have him for a while. He knows you are desperate and in a panic over him leaving. In many ways, he may even get a thrill out of the fact that he may have two women fighting over him. Let him go so that the ow gets the his moody self in her face all of the time. By letting him go, they spend more time together and start to see all of the warts and bad behavior because people tend to put on a good front when they first meet. If you step away, all of that "in your face time together will wear thing eventually because life really doesn't change because you still have to work, bills to pay and the normal day-to-day stuff continues". Give them all of that while you are working on you and taking care of your son.
You do realize that all of that FB garbage was put out there so that you would see it. They are rubbing this stuff in your face to get to you and make you so angry that you'll put him and his belongings out on the street. Don't help him. If he wants to leave, he's going to have to be a man and do it on his own.
Now about that puppy. If that puppy was purchased for him, then you need to consider several things...1) he needs to take the pup w/him because it is his; 2) if the puppy stays w/you, is your son going to be responsible for the pup's care; and 3) how do you propose to take care of a puppy when you were ungodly hours? Something has to give in this area because puppies require a lot of time and work. So, this is something you need to think about.
As we all have stated...focus on YOU! Try detaching more and stop asking him questions or attempting to make small talk w/him. If he wants to talk to you, he will find you and talk. He's a roommate for the time being. He fired you as a wife, so leave him be. You are not his mother...allow him to figure things out for himself and please stop making excuses for his behavior. He's the one out there having the affair...not you.
Sit quietly, the answers will reveal themselves when you least expect them to. The past is gone, the present is a gift and you need to focus on today, allow the future to reveal itself when it is ready.